Conventional PSTN telephony systems traditionally utilize conservative basic implementations and rigid national specifications. The common practice for many years was that of powerful monopolies, usually state owned, and it was characterized by total control on the network; according to the common practice, the best grade of service was required and ensured. However, looking at the power consumption issue at least in respect of “Plain Old Telephony Service” (POTS) lines, the conventional access systems are far from being optimal for the following reasons:                In many cases, a national specification refers to all lines uniformly, so all telephony lines are powered according to the principle “one size fits all”. (E.g., the ETSI EN 300 001 specification, summarizing the legacy analog subscriber characteristics for many European countries, or the Telcordia GR57CORE.001, GR303CORE.004, etc. that provides the legacy analog subscriber characteristics for North America.)        Only a recent new set of harmonized ETSI specifications, such as EG 201 188, ES 201 970, TS 102 971, etc. address different telephony line categories (i.e., Long lines or Short/Medium lines which should preferably be powered differently).        Usually, a common Battery rail (power bus and DC power source) feeds all telephony line cards and all lines on a specific card via their respective Subscriber Line Interface Circuits (SLICs, usually with the voltage of −48V, or with the voltage of −60V in some cases).        Inefficient old SLIC designs are still in use, designed to feed worst-case phones (that are rarely used these days). The same also applies to the Ringing voltage whose characteristics stem from old-fashioned and rarely found mechanical phones.        Even newer SLIC designs often use the conventional Constant-current mode that usually results in consuming constant power from the battery (power source) at the Off-hook state of the telephony line, regardless of the loop (line) length. (It should be kept in mind that the line is active—i.e., is power-feeding—at the Off-hook condition. However, a small yet constant quiescent power is consumed at any condition).        
Modern telephony line cards are usually fully programmable, allowing them to be configured to any specific set of national parameters such as: Transmission parameters, Tone and Ring patterns, Signaling, and some Loop characteristics on a per channel basis. However, the battery feed characteristics are exceptional in that they are not tailored for each channel individually because of excessive space and cost issues.
At the same time, in practice seldom is the case that most of the lines connected to a specific line card fall under the same category/have similar loop lengths. On the contrary, given today's high density cards and the normal distribution of loop length—it is fair to see that out of ˜100 subscriber lines per line-card statistically only 5-10% have the maximal length.
For example, a service provider may have a ‘Long’ (loop <2000Ω) and ‘Normal’ (loop <1500Ω) line categories, for which a Vbat Range value of −60V or −48V is chosen, respectively. Under the previous loop distribution assumptions, 90-95% of the lines will not use the predetermined VbatRange optimally, wasting 100-200 mW per active channel (It should be kept in mind that the channel is active—i.e., power-feeding—at the Off-hook condition). Upon multiplying this value by the Erlang figure (Erlang defines the statistical utilization of lines over a given period), one may obtain the average amount of power wasted in any access network. It should be kept in mind that access networks are permanently growing.
One of the major sources of PSTN power consumption is the SLICs' quiescent current consumed from Vbat during the Idle state (On-hook). Usually, Va-b(on-hook) is defined in an all-purpose National specification (typically −48V), however most Terminal Equipment elements (TEs) may function perfectly with a significantly lower voltage (i.e., some documents even state it, for example GR57 #5.3.8 requires: “on hook voltage>21Vdc”).
And yet another (minor) source of power, wasted in telephone networks, is found in the Ringing state. Usually, the Ringing Voltage Va-b(Ring) is defined in an all-purpose National specification (typically ˜80 Vrms), however most lines are relatively short and a significantly lower Ringing voltage value may be configured for them.
In contemporary access networks, Voice (telephony) line cards are housed in shelves and racks that are shared by several other services such as xDSL, PON, etc. They are usually aggregated into (in communication with) a common packet network such as IP or ATM, controlled by smart traffic-management tools and managed by a common Management system.
Modern telecom systems, especially in the access part of the network, have ever-growing power demands. This fact disagrees with the operators' need to lower their OPEX (operational expenses, i.e. electricity bills), and the vital general need of power efficiency (due to global warming, etc.). Claudio Bianco et al in the paper “Energy consumption trends in the Next Generation Access Network—a Telco perspective”, states that the electrical energy needed by the Telecom Italia's Network is more than 2,000,000,000,000 Wh(>2 TWh), representing nearly 1% of the total National energy demand, second user only to the National Railways. Electrical energy consumption for Telecom Italia's (TI) fixed network represents roughly 80% of the total TI network consumption. Similar numbers are actual for many other countries having a developed copper lines infrastructure (“Copper plant”).
US 2003/0202655 describes a system for managing power from a power source for at least one line powered network element in an access network. The system includes a power manager that establishes at least one power criterion for the at least one network element. Further, the system includes a power controller that is responsive to the power manager. The power controller and the power manager communicate to manage operation of the at least one network element based on the at least one power criterion and deals with cases where the power consumed by a line exceeds a predetermined threshold, or where another irregular situation takes place.
It should be noted that neither of the discussed prior art references takes care of regular situations, standard for the ordinary way of access networks' operation. Namely, neither of the solutions known to the applicant seems to take care of excessive power conventionally wasted during on-hook and/or off-hook states of telephony communication lines due to feed characteristics not optimized with respect to their line lengths (=line resistance).